Nitrogen pentoxide    N2O5     28   80     2    5


Note that the proportion of O by weight is in each case a
multiple of the first, 16. Also that the proportion by volume of
O is a multiple of that in the first compound. In this example
the N remains the same. If that had varied in the different
compounds, it would also havevaried by a multiple of the smallest
proportion. This is true in all compounds.

115. Law of Multiple Proportion.--Whenever one element combines
with another in more than one proportion, it always combines in
some multiple, one or more, of its least combining weight, or, if
a gas, of its least combining volume.

The least combining weight of an element is its atomic weight;
and it is this fact of a least combining weight that leads us to
believe the atom to be indivisible.

Apply the law in the case of P2O, P2O3, P2O5; in HClO, HClO2,
HClO3, HClO4, arranging the symbols, weights, and volumes in a
table, as above.

The volumetric proportions of each element in the oxides of
nitrogen are exhibited below.


_ + _ + _ = __
N + N + O = N2O

_ + _ + _ + _ = __
N + N + O + O = N2O2

_ + _ + _ + _ + _ = __
N + N + O + O + O = N2O3

_ + _ + _ + _ + _ + _ = __
N + N + O + O + O + O = N2O4

_ + _ + _ + _ + _ + _ + _ = __
N + N + O + O + O + O + O = N2O5

CHAPTER XXIV.

CARBON PROTOXIDE.

116. Preparation.

Experiment 72.--Put into a flask, of 200 cc., 5 g. of oxalic acid
crystals, H2C2O4, and 25 cc. H2SO4. Have the d.t. pass into a
solution of NaOH in a Woulff bottle (Fig. 31), and collect
the gas over water. Heat the flask slowly, and avoid inhaling the
gas.

117. Tests.

Experiment 73.--Remove a receiver of the gas, and try to light
the latter with a splinter. Is it combustible, or a supporter of
(C) combustion? What is the color of the flame? When the
combustion ceases, shake up a little lime water with the gas left
in the receiver. What gas has been formed by the combustion, as
shown by the test? See page 80. Give the reaction for the
combustion.

We have seen that H2SO4 has great affinity for H2O. Oxalic acid
consists of H, C, O in the right proportion to form H2O, CO2, and
CO. H2SO4 withdraws H and O in the right proportion to form
water, unites them, and then absorbs the water, leaving the C and
O to combine and form CO2 and CO. NaOH solution removes CO2 from
the mixture, forming Na2CO3, and leaves CO. Write both reactions.

118. Carbon Protoxide, called also carbon monoxide, carbonic
oxide, etc., is a gas, having no color or taste, butpossessing a
faint odor. It is very poisonous. Being the lesser oxide of C, it
is formed when C is burned in a limited supply of O, whereas CO2
is always produced when O is abundant. The formation of each is
well shown by tracing the combustion in a coal fire. Air enters
at the bottom, and CO2 is first formed. C + 2O = CO2. As this gas
passes up, the white-hot coal removes one atom of O, leaving CO.
CO2 + C - 2CO. At the top, if the draft be open, a blue flame
shows the combustion of CO. CO + O = CO2. The same reduction of
CO2 takes place in the iron furnace, and whenever there is not
enough oxygen to form CO2, the product is CO.

Great care should be taken that this gas does not escape into the
room, as one per cent has proved fatal. Not all of it is burned
at the top of the coal; and when the stove door is open, the
upper drafts should be open also. It is the most poisonous of the
gases from coal; hence the danger from sleeping in a room having
a coal fire.

119. Water Gas.--CO is one of the constituents of "water gas,"
which, by reason of its cheapness, is supplanting gas made from
coal, as an illuminator, in some cities. It is made by passing
superheated steam over red-hot charcoal or coke. C unites with
the O of H2O, forming CO, and sets H free, thus producing two
inflammable gases. C + H2O --? As neither of these gives much
light, naphtha is distilled and mixed with them in small
quantities to furnish illuminating power See page 183.

CHAPTER XXV.

CARBON DIOXIDE.

120. Preparation.

Experiment 74.--Put into a t.t., or a bottle with a d.t. and a
thistle-tube, 10 or 20 g. CaCO3, marble in lumps; add as many
cubic centimeters of H2O, and half as much HCl, and collect the
gas by downward displacement (Fig. 39). Add more acid as needed.
CaCO3 + 2 HCl = CaCl2 + H2CO3. H2CO3 = H2O + CO2. H2CO3 is a very
weak compound, and at once breaks up. By some, its existence as a
compound is doubted.

121. Tests.

Experiment 75.--(1) Put a burning and a glowing stick into the
t.t. or bottle. (2) Hold the end of the d.t. directly against the
flame of a small burning stick. Does the gas support combustion?
(3) Pour a receiver of the gas over a candle flame. What does
this show of the weight of the gas? (4) Pass a little CO2 into
some H2O (Fig. 32), and test it with litmus. Give the reaction
for the solution of CO2 in H2O.

Experiment 76.--Put into a t.t. 51 cc. of clear Ca(OH)2 solution,
i.e. lime water; insert in this the end of a d.t. from a CO2
generator (Fig. 32). Notice any ppt. formed. It is CaCO3. Let the
action continue until the ppt. disappears and the liquid is
clear. Then remove the d.t., boil the clear liquid for a minute,
and notice whether the ppt. reappears.

122. Explanation.

Ca(OH)2 + CO2 = CaCO3 + H2O. The curious phenomena of this
experiment are explained by the solubility of CaCO3 in water
containing CO2, and its insolu-bility in water, having no CO2.
When all the Ca(OH)3 is combined, or changed to CaCO3, the excess
of CO2 unites with H2O, forming the weak acid H2CO3, which
dissolves the precipitate, CaCO3, and gives a clear liquid. On
heating this, H2CO3 gives up its CO2, and CaCO3 is
reprecipitated, not being soluble in pure water.

Lime water, Ca(OH)2 solution, is therefore a test for the
presence of CO2. To show that carbon dioxide is formed in
breathing, and in the combustion of C, and that it is present in
the air, perform the following experiment:

Experiment 77.--(1) Put a little lime water into a t.t., and blow
into it through a piece of glass tubing. Any turbidity shows
what? (2) Burn a candle for a few minutes in a receiver of air,
then take out the candle and shake up lime water with the gas.
(3) Expose some lime water in an e.d. to the air for some time.

133. Oxidation in the Human System.--Carbon dioxide, or carbonic
anhydride, carbonic acid, etc., CO2, is a heavy gas, without
color or odor. It has a sharp, prickly taste, and is commonly
reckoned as poisonous if inhaled in large quantities, though it
does not chemically combine with the blood as CO does. Ten per
cent in the air will sometimes produce death, and five per cent
produces drowsiness. It exists in minute portions in the
atmosphere, and often accumulates at the bottom of old wells and
caverns, owing to its slow diffusive power. Before going down
into one of these, the air should always be tested by lowering a
lighted candle. If this is extinguished, there is danger. CO2 is
the deadly "choke damp" after a mine explosion, CH4 being
converted into CO2 and H2O; a great deal is liberated during
volcanic eruptions, and it is formed in breathing by the union of
O in the air with C in the system. This union of C and O takes
place in the lungs and in all the tissues of the body, even on
the surface. Oxygen is taken into the lungs, passes through the
thin membrane into the blood, forms a weak chemical union with
the red corpuscles, and is conveyed by them to all parts of the
system. Throughout the body, wherever necessary, C and H are
supplied for the O, and unite with it to form CO2 and H2O. These
are taken up by the blood though they do not form a chemical
union with it, are carried to the lungs, and pass out, together
with the unused N and surplus O. The system is thus purified, and
the waste must be supplied by food. The process also keeps up the
heat of the body as really as the combustion of C or P in O
produces heat. The temperature of the body does not vary much
from 99 degrees F., any excess of heat passing off through
perspiration, and being changed into other forms of energy.

If, as in some fevers, the temperature rises above about 105
degrees F., the blood corpuscles are killed, and the person dies.
During violent exercise much material is consumed, circulation is
rapid, and quick breathing ensues. Oxygen is necessary for life.
A healthy person inhales plentifully; and this element is one of
nature's best remedies for disease. Deep and continued
inhalations in cold weather are better than furnace fires to heat
the system. All animals breathe O and exhale CO2. Fishes and
other aquatic animals obtain it, not by decomposing H2O, but from
air dissolved in water. Being cold-blooded, they need relatively
little; but if no fresh water is supplied to those in captivity,
they soon die of O starvation.

124. Oxidation in Water.--Swift-running streams are clear and
comparatively pure, because their organic  impurities are
constantly brought to the surface and oxidized, whereas in
stagnant pools these impurities accumulate. Reservoirs of water
for city supply have sometimes been freed from impurities by
aeration, i.e. by forcing air into the water.

125. Deoxidation in Plants.--Since CO2 is so constantly poured
into the atmosphere, why does it not accumulate there in large
quantity? Why is there not less free O in the air to-day than
there was a thousand years ago? The answer to these questions is
found in the growth of vegetation. In the leaf of every plant are
thousands of little chemical laboratories; CO2 diffused in small
quantities in the air passes, together with a very little H2O,
into the leaf, usually from its under side, and is decomposed by
the radiant energy of the sun. The C is built into the woody
fiber of the tree, and the O is ready to be re-breathed or burned
again. CO2 contributes to the growth of plants, O to that of
animals; and the constituents of the atmosphere vary little from
one age to another. The compensation of nature is here well
shown. Plants feed upon what animals discard, transforming it

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